Optical monitoring of optical signals is very important in ensuring the correct operation of optical communications networks. The relevant standard for optical monitoring is set out in the International Telecommunications Union document ITU-T G.697 “Optical monitoring for Dense Wavelength Division Multiplexing Systems (DWDM) systems Series G: Transmission Systems and Media, Digital Systems and Networks Transmission Media Characteristics—Characteristics of optical components and subsystems”.
Optical monitoring points in a communications network typically comprise an optical splitter arranged to extract between 1% and 10% of the optical signal that is to be monitored, the extracted signal being provided to a monitoring port. All of the traffic carried by the optical signal being monitored is replicated in the extracted signal and is provided to the monitoring port. There is a resulting problem that live traffic is vulnerable to eavesdropping at the monitoring port and this presents a problem of communications network security.
WO 2011/103930 A1 describes a communications network element with a monitoring port. Optical signal transforming apparatus applies an optical transfer function (OTF) to a tapped signal. This preserves a spectral property of the tapped signal and applies a time-domain obfuscation to the tapped signal.